Interferon is one of the most important factors that are in charge of phylaxis or immune modulation. It has been already put to practical use as a therapeutic agent for hepatitis B and C and an immunotherapeutic agent for cancer. In particular, interferon is practically the only therapeutic agent available for hepatitis C. Interferon is a polypeptide having a molecular weight of about 20,000. It is produced by gene recombination or cell culture, and it can be administered only in the form of injection. What is desired is, accordingly, the development of an interferon inducer that can be orally administered.
Examples of known substances having interferon-inducing activity include double-stranded nucleic acids derived from viruses or other living organisms and high molecular weight polymers such as Poly(I)/Poly(C) or polycarboxylate. Double-stranded nucleic acids or high molecular weight polymers, however, are problematic in, for example, antigenicity, contamination by pathogenic microorganisms or biological stability. In addition, since they has a high molecular weight, development of oral preparations therefrom is difficult. Several substances, such as fluorenones, pyrimidinones, or anthraquinones have been examined as low molecular weight interferon-inducing substances (Mayer, G. D., et al.: Science, 1970, 169, 1214, Nichol, F. R. et al.: Antimicrob. Agents Chemother., 1976, 9, 433, Stringfellow, D. A., et al.: Antimicrob. Agents Chemother., 1991, 15, 111). Because of their low therapeutic effect or toxicity, however, development of pharmaceutical preparations therefrom was relinquished (Reiter, M. A., et al.: J. Leukocyte Biol. 1994, 55, 234). An imidazoquinoline derivative, R-837 (Imiquimod), is known as another low molecular weight interferon-inducing substance (EP 145,340). R-837, however, has low interferon-inducing activity, and the development thereof in the field of oral preparations was no longer performed due to its side effects. The present inventors also found that a specific purine derivative had interferon-inducing activity (WO 99-28321). Since these compounds had low water-solubility, they were not sufficient in terms of gastrointestinal absorption.
In contrast, helper T cells play major roles in immune responses. There are two types of helper T cells, i.e., Th1 cells and Th2 cells. Examples of cytokines produced upon the activation of Th1 cells are interleukin-2 (IL-2) and interferon-γ (IFN-γ). Examples of cytokines produced from Th2 cells are interleukin-4 (IL-4) and interleukin-5 (IL-5). Th1 cytokines activate macrophages, natural killer cells, or the like, and they are known to be mainly involved with cellular immunity such as phylaxis against viruses or bacteria. Th2 cytokines are involved with humoral immunity such as antibody production from B cells. In particular, IL-4 induces B cells to produce IgE antibodies and has actions of Th2 cell differentiation or proliferation. IL-5 is capable of activating eosinocytes, accelerating differentiation or proliferation, life lengthening, or the like. Accordingly, it often plays a major role in allergic inflammation. In fact, these Th2 cytokines are increased in lesions of patients having allergic inflammation such as asthma or atopic dermatitis with which Th2 cells are mainly involved. Steroid drugs are often used to treat these diseases. However, chronic administration of steroid drugs disadvantageously generates a variety of side effects such as diabetes, osteoporosis, adrenal disorder, or moon face. Since steroid drugs inhibitorily act against both T cells, i.e., Th1 cells and Th2 cells, they may cause infectious diseases as a result of inhibiting Th1 cells. Accordingly, pharmaceuticals that can selectively inhibit Th2 immune responses can be safe therapeutic agents for allergic diseases without causing infectious diseases.